With recent popularization of computers, an inkjet printer is widely used for printing letters or drawing an image on paper, film, cloth or the like not only at offices but also at homes.
The inkjet recording method includes a system of discharging a liquid droplet by applying a pressure from a piezoelectric element, a system of discharging a liquid droplet by generating a bubble in the ink under heat, a system of using an ultrasonic wave, and a system of discharging a liquid droplet by suction using an electrostatic force. The inkjet recording ink used therefor includes an aqueous ink, an oily ink and a solid (fusion-type) ink.
Among these inks, the aqueous ink is relatively superior to oily ink or solid (fusion-type) ink in view of possibility of satisfying all of production, handleability, odor, safety and the like and therefore, is predominating as the inkjet recording ink at present.
The coloring material used in such an inkjet recording ink is required to have high solubility in a solvent (ink medium), enable high-density recording, provide good color hue, exhibit excellent fastness to light, heat, air, water and chemicals, ensure good fixing to an image-receiving material and less blurring, give an ink having excellent storability, have high purity and no toxicity, and be available at a low cost. Various dyes and pigments for use in inkjet recording have been already proposed and are actually used, but a coloring material satisfying all of the above-described requirements is not yet found out at present.
For example, conventionally well-known dyes and pigments such as coloring material having a color index (C.I.) number can hardly satisfy all of those various properties required of the inkjet recording ink.
With respect to the black dye for inkjet ink, food black-type dyes, naphthol-based direct azo dyes, acidic azo dyes and the like have been heretofore widely known.
Representative examples of the food black-type dye include C.I. Food Black 1 and C.I. Food Black 2, and techniques of using such a dye for the inkjet black ink are described in [Patent Document 1] to [Patent Document 3].
As for the acidic azo dye, techniques of using C.I. Acid Black 2, 31, 52, 140, 187 or the like for the inkjet black ink are described in [Patent Document 4] to [Patent Document 6], and as for the direct azo dye, techniques of using C.I. Direct Black 9, 17, 38, 51, 60, 102, 107, 122, 142, 154, 168 or the like for the inkjet black ink are described in [Patent Document 7] to [Patent Document 9].
Usually, the black color hue can be hardly covered by a single dye and a short-wave dye is preferably used in combination.
With respect to this short-wave dye, direct azo dyes, acidic azo dyes and the like are also widely known and techniques of using C.I. Acid Yellow 17, 23, 49, 194 or the like or C.I. Direct yellow 86, 120, 132, 144 or the lie for the inkjet black ink are described in [Patent Document 10] to [Patent Document 13]. Despite these techniques, the above-described properties required of the inkjet ink is demanded to be more enhanced.
The present inventors have made studies on the inkjet ink using a dye, but an aqueous black ink has a problem of low image durability. Furthermore, unlike carbon black, almost all black dyes when each used alone cannot provide a black image such as letters with high printing quality and as described above, a mixture of multiple dyes differing in the absorption spectrum is generally used. In this case, dyes mixed differ in the discoloration rate or the absorption waveform of the dye having broad absorption characteristics is changed due to discoloration, as a result, the quality of the image containing black is seriously impaired. Therefore, fastness of an image is particularly important.
In addition, the water-soluble black ink must have absorption characteristics covering the wavelength region over a wide range and in many cases, a high solid content concentration is required, but this brings about a problem that the discharge property of the ink tends to be worsened. Improvement is necessary also in this point.
As a result of intensive investigations, it has been found that addition of an amide-based compound is effective. However, in the case where a low-volatile organic solvent is present together in the ink, if a large amount of an amide-based compound is used in combination, the image formed is found to be readily blurred under high-humidity condition and an amide-based or analogous compound free from bleeding is demanded.
[Patent Document 1]JP-A-2-36276 (the term “JP-A” as used hereinmeans an “unexamined published Japanesepatent application”)[Patent Document 2]JP-A-2-233782[Patent Document 3]JP-A-2-233783[Patent Document 4]JP-A-60-108481[Patent Document 5]JP-A-2-36277[Patent Document 6]JP-A-2-36278[Patent Document 7]JP-A-56-139568[Patent Document 8]JP-A-61-285275[Patent Document 9]JP-A-3-106974[Patent Document 10]JP-A-7-97541[Patent Document 11]WO97/16496[Patent Document 12]JP-A-10-158560[Patent Document 13]JP-A-11-12514
An object of the present invention is to solve the problems of the black ink and the like for inkjet recording described in the Background Art, more specifically, to provide an inkjet recording ink, particularly, black ink, ensuring excellent discharge stability even after aging of the ink and also exhibiting excellent weather resistance of causing no aging deterioration of the image quality.